1. Field of Invention
The present invention relates to a fixing apparatus for fixing images formed on sheet and an image forming apparatus provided with this fixing apparatus.
2. Description of Related Art
In general, an image forming apparatus as an electrophotographic system (printer, copying machine, facsimile or the like) is provided with a fixing apparatus for applying heat and pressure to a sheet to fix a toner image transferred to the sheet. This fixing apparatus includes a heating unit for heating and melting toner on a sheet and a pressing unit for pressing the sheet against the heating unit.
For example, this type of fixing apparatus has been implemented as follows. Japanese Patent Published Application No. 6-250560 discloses a fixing apparatus having a nip portion which is formed between a fixing roller and a part of an endless belt wound on a plurality of rollers. The fixing apparatus also includes a pressure roller located in contact with the fixing roller through the endless belt from the inside of the endless belt at the exit of the nip portion. The fixing apparatus prevents displacement of images by exerting a braking force on the endless belt conveyed on the pressure roller in order to remove the difference in the conveyance speed between the pressure roller and the fixing roller.
On the other hand, Japanese Patent Published Application No. 09-138598 discloses a fixing apparatus provided with a pressure roller and a plurality of rollers on which a fixing belt is wound, and applies a braking force to the fixing belt by rotationally driving one of the plurality of rollers, and rotationally driving the other roller in the direction to inhibit the rotation of the one roller. In accordance with this configuration of this fixing apparatus, the fixing belt is stretched between the one roller and the other roller so that a nip portion is formed between the stretched portion of the fixing belt and the pressure roller to tightly contact the fixing belt with the outer peripheral surface of the pressure roller without gaps.
However, in the case where a plurality of sheets of a cardboard or a paper having a rough surface are successively passed through the fixing apparatus described in Japanese Patent Published Application No. 6-250560 or Japanese Patent Published Application No. 9-138598, the surface of the fixing belt is made rough at opposite side edges of these thick or rough sheets. If a large size sheet is passed through the fixing apparatus for fixing an image thereon after the thick or rough sheets, a gloss line may appear on the fixed image corresponding to the rough surface of the fixing belt such that the image looks less shiny in the gloss line than in other portions.
In order to solve this problem, for example, it may be effective to apply a driving force to one of a fixing and a back side member which are formed of the fixing belt and the rollers, and apply a braking force to the other, and recover the rough surface of the fixing side member and back side member by idle-rotating the fixing side member and the back side member for a certain time in a non-fixing period in which the nip width is made shorter than that in a fixing period.
However, depending upon the strength of the braking force, the motor driven to apply the braking force may be heated and therefore it is difficult to continuously perform the recovering process in an appropriate manner.